The invention is directed to a mass for an emission electrode which is to be operated at a high temperature, in which the mass comprises a power mixture of thorium dioxide and aluminum oxide, and to a method for the production of such a mass.
German OS No. 1 589 111 discloses an electrode for electron tubes, which comprises a tungstem rod, to which is added thorium dioxide. It is also provided with a heating coil, likewise formed from tungsten, which is operatively to suitably heat the electrode. To facilitate the emission of the electrons, the tip of the tungsten rod and the coil are coated with a mixture of thorium dioxide powder and thorium powder, a bonding agent being employed to enable such powder mixture to adhere to the focal spot of the electrode. The powder deposit is then transformed, by means of an arc discharge at approximately 2000.degree.C, to form a ceramic mass which then firmly adheres to the tungsten wire and to the electrode pin. The bonding agent employed evaporates when the coating mass fuses. Such coating mass has a relatively good electron emission capacity but, in contrast to other previously known electrodes for tubes, has a relatively low mechanical stability. Tungsten when mixed with thorium dioxide and thorium is extremely brittle, such brittleness increasing with the quantity of thorium added to the tungsten. Likewise, the bonding strength and stability of a mixture composed of thorium and thorium oxide may be subjected to only a limited thermal load. If the temperature rises to above the melting point of thorium (1845.degree.C) the mass will readily become molten and a dripping of the electrode material will take place. Further, in the solid state the mass has a very smooth and flat surface structure.
Additionally, German Pat. No. 1 806 856 discloses a non-fusible electrode comprising tungsten with thorium additives for use in so-called tungsten inert gas welding (TIG). In order to increase the electron-emission capacity, such electrode is provided with a coating layer of thorium dioxide, which is applied to the electrode tip by fuse-sintering. In effecting such operation, the tip of the electrode is first coated with thorium dioxide powder and is then slowly heated until the thorium dioxide melts. Thereupon, at an initially low current value, an arc is ignited and then by increasing the electrode distance and greatly increasing the current to well above the normal operating range of the electrode, the thorium dioxide is sintered onto the surface of the electrode tip. Coating layers of this type also have a good electron-emission capacity.
Although such electrodes have proved valuable in actual practice, they are extremely sensitive to mechanical loads, for example, such as may readily occur during welding in the event of contact with the workpiece, whereby the mass falls out and the electrode becomes unserviceable.
In the above referred to patent, it is proposed that other metal oxide be added to the coating mass, in this instance of thorium dioxide, to facilitate the sintering process. For this purpose there is employed a metal oxide whose melting point is at a considerably lower temperature than the thorium dioxide. However, it has been proven that when such metal oxides are employed, either the mechanical and/or the thermal load capacity, as well as the electron-emission capacity will drop. The two metal oxides form an eutectic so that even when the electrode is subject to a slight overload, as previously described, the mass becomes molten, resulting in the electrode failing practically at a single stroke.